Page 478 - Petrophysics

446 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

The dimensionless pressure of a well producing at a constant rate in the
infinite-acting portion of the pressure vs. time curve, i.e., during unsteady
state, is approximated by:
2 )
1 5.92 x 10-*kt
2 @CLctrw (7.88)

In this case, the pressure drop, Ap, in Equation 7.78 is actually (pi - pw),
where pi is the initial reservoir pressure. Equation 7.88 is applicable only
if (0.0002637 kt/@yctr:) > 100.

EXAMPLE

A Cartesian plot of pressure data recorded during a constant rate well
test in a water driven oil reservoir yielded a pseudosteady-state straight
line with slope -0.26 psihr. Other pertinent reservoir and well data
include the following:

A = 7.72 acres qs, = 350 STB/D
h=49ft Be = 1.136 bbl/STB

6
@ = 0.23 CE = 17 x 10- psi-'

Determine the drainage boundary index f. Is the pressure at the
drainage boundary constant?

SOLUTION

The drainage boundary index f is calculated from a rearranged form of
Equation 7.87:

(7.89)

where the drainage pore volume is:

Vp = Ah@ = 7.72 x 43,560 x 49 x 0.23 = 3.79 x lo6 ft3

The boundary index is:

17 x lo6 x 3.79 x lop6
(-0.26) = 0.82
f=l+ 1
0.234 x 350 x 1.136